The present invention relates to a method of detecting defects formed on an optical record medium for use in an optical information recording and reproducing apparatus and a device for carrying out the method.
In an apparatus for recording and/or reproducing information by light beam, the recording of information is performed by rotating a disc covered with a thermoplastic film or a metal film (hereinafter referred to as a record medium), irradiating laser beam modulated according to information and focused to form a fine spot having 1.mu. diameter onto the record medium and forming a finely changed portion (usually referred to as pit) on the surface of the record medium.
As shown in FIG. 1a, for example, it is assumed that a pulse-shaped writing signal modulated with information is a laser drive signal a. When the signal a has a power larger than a specific writing threshold level for the record medium, the surface portion of the record medium is subjected to a variation or change such as dissolution, deformation or crysterization so that a reflected light outputs b (hereinafter referred to as RF signal) can be obtained from light receiving elements which detect light reflected from the surface of the record medium. As seen from the waveform of the RF signals b, during recording time the RF signals have an amplitude of maximum peak d from the beginning time of information writing to the time at which the surface of the record medium begins to change, and then the intensity of the RF signal decreases by an interface generated due to the formation of the pit and the decrease of reflection factor and thus stationary outputs e are generated after completion of writing. In the DRAW method (Direct Read After Write method), furthermore, even after ending of writing pulse the laser generates weak light beam so that the reflected light outputs have an amplifier of minimum peak f at the portion at which pits are already formed, and then stationary outputs g can be obtained.
As a defect formed on the surface of the record medium, generally, there are two types of defects, one being a defect formed on the pit portion and the other being a defect formed on the groove portion. As shown in FIG. 2, the defect formed on the pit portion appears on the RF signal as waveforms of, for example, i.sub.1, i.sub.2 and i.sub.3.
The defect formed on the groove portion appears on the RF signal as waveforms of h.sub.1 and h.sub.2.
Means for detecting such defects is disclosed in Japanese Patent Laid-open No. 212629/82. These prior arts decide whether the writing is normal or abnormal by monitoring dynamic change and change in time of the reflected light quantity at the time of writing.
These defect detections can be applied to only the case of low signal recording density and can not be applied to the case of high density signal recording of 1 MH.sub.z. These defect detections have bad precision. This causes not only over detection but also under detection so that recording density is decreased and efficiency of the record medium is deteriorated.